Current frequency plans for frequency converters can be very complex due to the desired avoidance of creating interfering signals or spurs. One of the most important aspects in any radio frequency transceiver design is a properly chosen frequency plan. The basis for frequency planning is drawn from the well-known formula describing the creation of a spurious signal as follows:fsp=mfrf±nflowhere fsp is the spurious output frequency or spur, frf is the frequency of the RF signal and flo is the local oscillator frequency.
Frequency plans have become quite complicated for modern communication systems such as multi-band cellular phones and transceivers in wireless networks. The design of these communication devices involve finding optimum solutions to several issues. These issues include defining the proper intermediate frequency to eliminate in band spurious signals, minimizing the number of IF filters, and determining the frequency swing for the voltage controlled oscillator. As a result there is a need for a frequency tunable filter for avoiding the creation of these undesired spurs, thus eliminating design difficulties, obviating the need for complex multiple frequency conversions and the associated deleterious effects on performance and cost ineffectiveness that accompanies them.
Furthermore, with limited spectrum resources serving greater and greater numbers of users the need for precise control of bandwidth and center frequency become even more important in the crowded airwaves. Bandpass, band reject and other types of filters composed of electronic components experience bandwidth variance as well as center frequency creep as a result of, among other things, temperature changes and the associated effect on the electrical characteristics of the electronic components. Typical transceivers deployed with these types of filters either operate with the aforementioned performance degradation or employ complicated and expensive compensation circuitry to combat this problem. Thus there is a need for a frequency tunable filter with temperature compensation to maintain a constant bandwidth and steadfast center frequency without sacrificing performance or cost effectiveness.
Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art to provide a novel system and method for a frequency tunable filter to provide at an output terminal an output signal at one of a plurality of predetermined frequencies comprising, input circuitry for providing an input signal that is at least in part a function of a dynamic parameter, a processor for providing a control signal as a function of said input signal; and at least one adjustable resonance circuit capable of providing a filter signal at one of a plurality of predetermined frequencies in response to said control signal, wherein said filter signal is applied to said output terminal to thereby provide said output signal at one of a plurality of predetermined frequencies.
It is also an object of the present invention to provide a novel improved method for reducing spurious signals from a composite signal created from mixing a plurality of signals by filtering the composite signal. An improvement in one embodiment can include the steps of selecting a control signal from a set of predetermined control signals, tuning the frequency of a filter based on the selected control signals and filtering the composite signal with the tuned filter.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.